1. Field of the Invention
This invention relates to the construction of a recombinant African Swine Fever Virus (ASFV) live attenuated candidate strain vaccine for the highly virulent Georgia 2007 isolate ASFV-G. The vaccine comprises the ASFV-G Δ9GL, a recombinant ASFV-G modified by deleting a large portion of the 9GL gene.
2. Description of the Relevant Art
African Swine Fever (ASF) is a contagious viral disease of swine. The causative agent, ASF virus (ASFV), is a large enveloped virus containing a double-stranded DNA genome of approximately 190 kilobase pairs. ASFV shares aspects of genome structure and replication strategy with other large double-stranded DNA viruses, including the Poxviridae, Iridoviridae and Phycodnaviridae (Costard et al. 2009. Phil. Trans. Royal Soc. B 364:2683-2696). ASFV infections in domestic pigs are often fatal and are characterized by fever, hemorrhages, ataxia and severe depression. However, the course of infection varies, ranging from highly lethal to sub-clinical, depending on host characteristics and the particular virus strain (Tulman et al. 2009. Curr. Top. Microbiol. Immunol. 328:43-87).
Currently, the disease is endemic in more than twenty sub-Saharan African countries. In Europe, ASF is still endemic on the island of Sardinia (Italy) and new outbreaks have been declared in the Caucasus region since 2007, affecting Georgia, Armenia, Azerbaijan and Russia. Isolated outbreaks have been recently reported in Ukraine, Belarus, Lithuania, Latvia and Poland, posing the risk of further dissemination into neighbouring countries. The epidemic virus, ASFV Georgia 2007/1, is a highly virulent isolate belonging to the genotype II (Chapman et al. 2011. Emerging Infect. Dis. 17:599-605).
At present, there is no vaccine available for ASF and disease outbreaks are controlled by animal quarantine and slaughter. Attempts to vaccinate animals using infected cell extracts, supernatants of infected pig peripheral blood leukocytes, purified and inactivated virions, infected glutaraldehyde-fixed macrophages, or detergent-treated infected alveolar macrophages failed to induce protective immunity (Coggins, L. 1974. Prog. Med. Virol. 18:48-63; Forman et al. 1982. Arch. Virol. 74:91-100; Kihm et al. 1987. In: African Swine Fever, Becker, Y. (ed), Martinus Nijhoff, Boston, pp 127-144; Mebus, C. A. 1988. Adv. Virus Res. 35:251-269). Homologous protective immunity does develop in pigs surviving viral infection. Pigs surviving acute infection with moderately virulent or attenuated variants of ASFV develop long-term resistance to homologous, but rarely to heterologous, virus challenge (Hamdy and Dardiri. 1984. Am. J. Vet. Res. 45:711-714; Ruiz-Gonzalvo et al. 1981. In: FAO/CEC Expert Consultation in ASF Research, Wilkinson, P. J. (ed), Rome, pp 206-216). Pigs immunized with live attenuated ASF viruses containing engineered deletions of specific ASFV virulence-associated genes were protected when challenged with homologous parental virus. Specifically, individual deletion of UK (DP69R), 23-NL (DP71L), TK (A240L) or 9GL (B119L) genes from the genomes of pathogenic ASF viruses (Malawi Lil-20/1, Pretoriuskop/96/4, and E70) markedly attenuated the virus in swine and the animals immunized with these attenuated viruses were protected against challenge with homologous virus (Moore et al. 1998. J. Virol. 72:10310-10315; Lewis et al. 2000. J. Virol. 74:1275-1285; Zsak et al. 1996. J. Virol. 70:8865-8871; Zsak et al. 1998. J. Virol. 72:1028-1035). In particular, deletion of 9GL (B119L) in highly virulent ASFV isolates Malawi Lil-20/1 and Pretoriuskop/96/4 resulted in complete attenuation of these viruses in swine (Lewis et al., supra; Neilan et al. 2004. Virol. 319:337-342). Administration of Malawi Lil-20/1 or Pretoriuskop/96/4 Δ9GL mutants to pigs via IM injection at a relatively high virus dose (104 HAD50 [50% hemadsorbing dose]) did not induce clinical signs, with all animals surviving the infection. Furthermore, IM inoculation of pigs with these viruses even at a relatively low dose (102 HAD50) induced protection against challenge with virulent Malawi Lil-20/1 virus (Lewis et al., supra). These observations constitute the only experimental evidence describing the rational development of an effective live attenuated virus against ASFV.
Although a deletion within the 9GL (B119L) gene from ASF Malawi Lil-20/1 and Pretoriuskop/96/4 and E70 had resulted in attenuated viruses effective for protection from pathogenic parental ASFV, the modified viruses did not protect against heterologous ASFV strains. Thus, there is a need for an effective live attenuated vaccine for the highly virulent ASFV Georgia 2007 isolate, ASFV-G, for which there is no vaccine candidate.